Swift registry metadata: preserve more JSON fields and accept empty metadata (#37254) (#37261)

Backport #37254

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

modules/test/utils.go

@@ -5,6 +5,7 @@ package test
 
 import (
 	"archive/tar"
+	"archive/zip"
 	"bytes"
 	"compress/gzip"
 	"io"
@@ -97,6 +98,17 @@ func WriteTarArchive(files map[string]string) *bytes.Buffer {
 	return WriteTarCompression(func(w io.Writer) io.WriteCloser { return util.NopCloser{Writer: w} }, files)
 }
 
+func WriteZipArchive(files map[string]string) *bytes.Buffer {
+	buf := &bytes.Buffer{}
+	zw := zip.NewWriter(buf)
+	for name, content := range files {
+		w, _ := zw.Create(name)
+		_, _ = w.Write([]byte(content))
+	}
+	_ = zw.Close()
+	return buf
+}
+
 func WriteTarCompression[F func(io.Writer) io.WriteCloser | func(io.Writer) (io.WriteCloser, error)](compression F, files map[string]string) *bytes.Buffer {
 	buf := &bytes.Buffer{}
 	var cw io.WriteCloser